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<a name="Defining-Predicates"></a>
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Previous: <a href="Machine_002dIndependent-Predicates.html#Machine_002dIndependent-Predicates" accesskey="p" rel="prev">Machine-Independent Predicates</a>, Up: <a href="Predicates.html#Predicates" accesskey="u" rel="up">Predicates</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Defining-Machine_002dSpecific-Predicates"></a>
<h4 class="subsection">16.7.2 Defining Machine-Specific Predicates</h4>
<a name="index-defining-predicates"></a>
<a name="index-define_005fpredicate"></a>
<a name="index-define_005fspecial_005fpredicate"></a>

<p>Many machines have requirements for their operands that cannot be
expressed precisely using the generic predicates.  You can define
additional predicates using <code>define_predicate</code> and
<code>define_special_predicate</code> expressions.  These expressions have
three operands:
</p>
<ul>
<li> The name of the predicate, as it will be referred to in
<code>match_operand</code> or <code>match_operator</code> expressions.

</li><li> An RTL expression which evaluates to true if the predicate allows the
operand <var>op</var>, false if it does not.  This expression can only use
the following RTL codes:

<dl compact="compact">
<dt><code>MATCH_OPERAND</code></dt>
<dd><p>When written inside a predicate expression, a <code>MATCH_OPERAND</code>
expression evaluates to true if the predicate it names would allow
<var>op</var>.  The operand number and constraint are ignored.  Due to
limitations in <code>genrecog</code>, you can only refer to generic
predicates and predicates that have already been defined.
</p>
</dd>
<dt><code>MATCH_CODE</code></dt>
<dd><p>This expression evaluates to true if <var>op</var> or a specified
subexpression of <var>op</var> has one of a given list of RTX codes.
</p>
<p>The first operand of this expression is a string constant containing a
comma-separated list of RTX code names (in lower case).  These are the
codes for which the <code>MATCH_CODE</code> will be true.
</p>
<p>The second operand is a string constant which indicates what
subexpression of <var>op</var> to examine.  If it is absent or the empty
string, <var>op</var> itself is examined.  Otherwise, the string constant
must be a sequence of digits and/or lowercase letters.  Each character
indicates a subexpression to extract from the current expression; for
the first character this is <var>op</var>, for the second and subsequent
characters it is the result of the previous character.  A digit
<var>n</var> extracts &lsquo;<samp>XEXP&nbsp;(<var>e</var>,&nbsp;<var>n</var>)<!-- /@w --></samp>&rsquo;; a letter <var>l</var>
extracts &lsquo;<samp>XVECEXP&nbsp;(<var>e</var>,&nbsp;0,&nbsp;<var>n</var>)<!-- /@w --></samp>&rsquo; where <var>n</var> is the
alphabetic ordinal of <var>l</var> (0 for &lsquo;a&rsquo;, 1 for &rsquo;b&rsquo;, and so on).  The
<code>MATCH_CODE</code> then examines the RTX code of the subexpression
extracted by the complete string.  It is not possible to extract
components of an <code>rtvec</code> that is not at position 0 within its RTX
object.
</p>
</dd>
<dt><code>MATCH_TEST</code></dt>
<dd><p>This expression has one operand, a string constant containing a C
expression.  The predicate&rsquo;s arguments, <var>op</var> and <var>mode</var>, are
available with those names in the C expression.  The <code>MATCH_TEST</code>
evaluates to true if the C expression evaluates to a nonzero value.
<code>MATCH_TEST</code> expressions must not have side effects.
</p>
</dd>
<dt><code>AND</code></dt>
<dt><code>IOR</code></dt>
<dt><code>NOT</code></dt>
<dt><code>IF_THEN_ELSE</code></dt>
<dd><p>The basic &lsquo;<samp>MATCH_</samp>&rsquo; expressions can be combined using these
logical operators, which have the semantics of the C operators
&lsquo;<samp>&amp;&amp;</samp>&rsquo;, &lsquo;<samp>||</samp>&rsquo;, &lsquo;<samp>!</samp>&rsquo;, and &lsquo;<samp>?&nbsp;:<!-- /@w --></samp>&rsquo; respectively.  As
in Common Lisp, you may give an <code>AND</code> or <code>IOR</code> expression an
arbitrary number of arguments; this has exactly the same effect as
writing a chain of two-argument <code>AND</code> or <code>IOR</code> expressions.
</p></dd>
</dl>

</li><li> An optional block of C code, which should execute
&lsquo;<samp>return&nbsp;true<!-- /@w --></samp>&rsquo; if the predicate is found to match and
&lsquo;<samp>return&nbsp;false<!-- /@w --></samp>&rsquo; if it does not.  It must not have any side
effects.  The predicate arguments, <var>op</var> and <var>mode</var>, are
available with those names.

<p>If a code block is present in a predicate definition, then the RTL
expression must evaluate to true <em>and</em> the code block must
execute &lsquo;<samp>return&nbsp;true<!-- /@w --></samp>&rsquo; for the predicate to allow the operand.
The RTL expression is evaluated first; do not re-check anything in the
code block that was checked in the RTL expression.
</p></li></ul>

<p>The program <code>genrecog</code> scans <code>define_predicate</code> and
<code>define_special_predicate</code> expressions to determine which RTX
codes are possibly allowed.  You should always make this explicit in
the RTL predicate expression, using <code>MATCH_OPERAND</code> and
<code>MATCH_CODE</code>.
</p>
<p>Here is an example of a simple predicate definition, from the IA64
machine description:
</p>
<div class="smallexample">
<pre class="smallexample">;; <span class="roman">True if <var>op</var> is a <code>SYMBOL_REF</code> which refers to the sdata section.</span>
(define_predicate &quot;small_addr_symbolic_operand&quot;
  (and (match_code &quot;symbol_ref&quot;)
       (match_test &quot;SYMBOL_REF_SMALL_ADDR_P (op)&quot;)))
</pre></div>

<p>And here is another, showing the use of the C block.
</p>
<div class="smallexample">
<pre class="smallexample">;; <span class="roman">True if <var>op</var> is a register operand that is (or could be) a GR reg.</span>
(define_predicate &quot;gr_register_operand&quot;
  (match_operand 0 &quot;register_operand&quot;)
{
  unsigned int regno;
  if (GET_CODE (op) == SUBREG)
    op = SUBREG_REG (op);

  regno = REGNO (op);
  return (regno &gt;= FIRST_PSEUDO_REGISTER || GENERAL_REGNO_P (regno));
})
</pre></div>

<p>Predicates written with <code>define_predicate</code> automatically include
a test that <var>mode</var> is <code>VOIDmode</code>, or <var>op</var> has the same
mode as <var>mode</var>, or <var>op</var> is a <code>CONST_INT</code> or
<code>CONST_DOUBLE</code>.  They do <em>not</em> check specifically for
integer <code>CONST_DOUBLE</code>, nor do they test that the value of either
kind of constant fits in the requested mode.  This is because
target-specific predicates that take constants usually have to do more
stringent value checks anyway.  If you need the exact same treatment
of <code>CONST_INT</code> or <code>CONST_DOUBLE</code> that the generic predicates
provide, use a <code>MATCH_OPERAND</code> subexpression to call
<code>const_int_operand</code>, <code>const_double_operand</code>, or
<code>immediate_operand</code>.
</p>
<p>Predicates written with <code>define_special_predicate</code> do not get any
automatic mode checks, and are treated as having special mode handling
by <code>genrecog</code>.
</p>
<p>The program <code>genpreds</code> is responsible for generating code to
test predicates.  It also writes a header file containing function
declarations for all machine-specific predicates.  It is not necessary
to declare these predicates in <samp><var>cpu</var>-protos.h</samp>.
</p>
<hr>
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